Molecular sieve separation processes



number of pores of uniform size.

United States Patent Ofilice 3,33,Z3l Patented Feb. 7, 1967 MQLECULARSIEVE SEPARA'HQN PRUCESSE Clive Leonard Hicks and Robert MarshallMacnab,

Sunbury-on-Tharnes, Middlesex, England, assignors to The BritishPetroleum Company Limited, London,

England, a corporation of England No Drawing. Filed Apr. 15, 1964, Ser.No. 360,075 Claims priority, application Great Britain, May 13, 1963,

6 Claims. (Cl. 260-676) This invention relates to the separation ofstraightchain hydrocarbons from hydrocarbon mixtures, particularlypetroleum fractions, by means of A. molecular sieves. More especiallythis invention is concerned with the separation of n-paraflin productsof very high purity, i.e., greater than 99%, and, particularly, greaterthan 99.9% purity.

It is well known that certain natural and synthetic zeolites have theproperty of preferentially absorbing certain types of hydrocarbons.These zeolites, known as molecular sieves, have crystalline structurescontaining a large In different zeolites, these pores may vary from 4 A.to 15 A. or more in diameter, but in any one zeolite the pores will beof substantially uniform size.

It had previously been proposed to treat petroleum fractions rangingfrom gasoline to gas-oils and higher with molecular sieves having porediameters ranging from 4 A. to 15 A. In order to separate straight-chainhydrocarbons from branched-chain and/ or cyclic hydrocarbons a molecularsieve having pore diameters of 5 A. is suitable. Such a process may beused to recover a denormalised fraction, for example gasoline of higheroctane number due to the removal of low octane normal paraffins. Theabsorbed straight-chain material may also be recovered as a product ifdesire-d. For example, a process suitable for separating normalparafiins from petroleum I fractions boiling within the C -C range isdescribed in our co-pending U.S. patent application Serial No. 168,-981, filed January 26, 1962, now U.S. Patent No. 3,201,-

490, said application corresponding to UK. application No. 3965/61.

The above application describes an isothermal process comprisingcontacting a C C fraction with a 5 A. molecular sieve in a first stageto absorb straight chain hydrocarbons, contacting the sieve with apurging medium in a second stage to remove material adsorbed on thesurface of the sieve or held interstitially between sieve particles,desorbing the absorbed hydrocarbons in a third stage using eithern-butane or n-pentane as desorbing medium, all three stages being in thevapour phase, the pressure in the desorption stage being equal to, orgreater than, the pressure in the absorption stage. Suitable absorptiontemperatures and pressures lie within the ranges 300-450 C. preferably350390 C., and 0300 p.s.i.g., preferably 100-150 p.s.i.g.

In a process such as that described in our above-mentioned U.S.application it is possible to obtain a n-parafiin product of greaterthan 95% purity. For example, a straight-run petroleum fractioncontaining 22% weight of n-paraflins may be treated by the above processand a pro-duct containing 97% weight of n-paratfins recovered. Whilethis is a relatively pure product for many purposes,

if it is used as a feedstock for a subsequent process in which onlypartial reaction of n-parafiins takes place, thus necessitating recycleof unreacted n-parafiins, the quantity of non-paraffinic impurity in thefeedstock will quickly build up. For example, if the process requiresunreacted feedstock to be recycled at the rate of 50% weight andrequires a n-paraffin feedstock purity of weight minimum, if theoriginal feedstock purity is 97% weight, the impurity will build up tomore than the maximum 10% permissible after only five process cycleseven allow ing for a 50% make-up with 97% pure feedstock after eachprocess cycle. If, however, a n-parafiin feedstock of 99.9% weightpurity was employed, under the same conditions 198 process cycles couldbe performed before the purity of the feedstock dropped to the minimum90% permissible.

According to the present invention a process for the production ofn-parafiin fractions having a purity of at least 99% weight, preferablyat least 99.9% weight, comprises treating a petroleum fraction in knownmanner such as that shown in our co-pending application Serial No.168,981, now US. Patent No. 3,201,490, with a 5 A. molecular sieve torecover an intermediate product containing at least 90% and preferablyat least weight of n-parafiins and reprocessing with a 5 A. molecularsieve to give a fraction containing less than 1%, preferably less than0.1%, by weight of isoparaffins and naphthenes, wherein either theintermediate product or the low isoparaflin/naphthene fraction isdearomatized, and recovering a n-parafiin product having a purity of atleast 99%, preferably at least 99.9%, by weight.

Although either the intermediate product or the reprocessed fraction maybe dearomatized it is preferred to dearomatize the reprocessed fraction.Any suitable dearomatizing operation may be employed, for example,hydrogenation, adsorption on silica gel, alumina or bauxite, oleumwashing being preferred, however.

It is common practice to improve the purity of a product by simplyreprocessing it. In a molecular sieve separation process, however, thissimple technique, although it would result in an improvement in purity,would not give a product of the required 99%-+ purity. For example, inthe process described in the above mentioned co-pending application itmight be expected that the purity of the n-paraflin product wouldincrease by a constant proportion on reprocessing so that if the firsttreatment of a feedstock containing 22% n-paraflins yielded a productcontaining 97% n-paraffins, reprocessing would be expected to yield aproduct containing 99.88% nparaffins. This would be expected if theimpurity is caused by traces of material of feed composition not removedfrom the sieve surface during the purge stage. In practice, however, theimprovement in purity is not as great. This is due to the fact that somecomponents of the feedstock, particularly aromatics, are selectivelysorbed on the surface of the molecular sieve during the absorption stageand are not removed during the purge stage although they are removedwith the product during the desorption stage. The impurity is notentirely due to this effect, however, otherwise reprocessing the productwould give no improvement in purity whatever as the surface adsorptionand desorption of aromatics would merely repeat itself. In fact theimpurities in the n-paraffin product are partly due to this surfaceadsorption and partly due to hold-up of feed material between sieveparticles. This is illustrated in Table 1 below:

prising contacting a petroleum fraction boiling in the C -C range with a5 A. molecular sieve at a temperature in the range 3004-50 C. and at apressure of -300 p.s.i.g. to produce an intermediate product containingTable 1 at least 95%, by weight, of n-parafiins, subjecting theintermediate product to a dearomatization operation, contacting thedearomatized intermediate product with a Original Oncc- Reproc- 5 A.molecular sieve at a temperature in the range 300- stream 5%}; 313311 333 450 C. and at a pressure of 0-300 p.s.i.g. to produce a fractioncontaining less than 1.0% by weight of iso- Normal Paraflin Content,percent wt 21.7 93.1 98.8 Parafiins and naphthenes, and recovering an'Parafi1n product therefrom having a purity of at least 99%.

H E}? 2. A process as claimed in claim 5 wherein the dearomatization iscarried out by oleum washing.

2}? NM 3. A process as claimed in claim 5 wherein the de- T t u Ir q N mt aromatization' is carried out by adsorption on silica gel. 5&3 (l2 4.A process for the production of n-parafiin fractions having a purity ofat least 99% by weight comprising Proportion of Aromatic Impurity,contacting a petroleum fraction boiling in the C ,C percent 26 51 83 20range with a 5 A. molecular sieve at a temperature in the range 300450C. and at a pressure of O300 p.s.i.g. to produce an intermediate productcontaining at least It is seen that although there is continuousimprove- 95% by weight of n-paralfins, contacting the intermediate mentin the purity of the product, this improvement beproduct with a 5 A.molecular sieve at a temperature comes proportionally smaller and theimpurity becomes i h range 300n450 C, d at a pressure of 0-300progressively more aromatic in chafiactef, the isoparafin/ p.s.i.g. toproduce a low isoparaffin/naphthene fraction naphlhene impurities beingProportlonany reducedcontaining less than 1.0% by weight of isoparafiinsand By aromatlfls from the f naphthenes, subjecting the lowisoparaflin/naphthene frac- PIPCCSSmg a considerable,"mprovement punfy ition to a dearomatizing operation, and recovering a ntained as comparedwith simple reprocessing as 1nd1cated 30 parafiin product therefromhaving a purity of at least 99% I by the followmg example 5. A processas claimed in claim 4 wherein the de- Example aromatization is carriedout by volume washing. A petroleum fraction boiling within the range 250to A Process as claimed in Claim @Wherein the 290 was treated i a 5 Amo1eu1ar Sieve b h aromatization is carried out by adsorption On silicagel.

Tem- Pressure. Duration, Stage Feedstock pegatcure, p.s.i.g. Rate Minues Absorption 250-290 C. Gas oil 0. 6 LHSV 6 Nitrogen 120 GHSV PurgeNitrogen 380 125 120 GHSV 6 Desorption Normal Pentane 1. 0 LHSV 12process described in co-pending application No. 3,965/ 61, ReferencesCited by the Examiner the process conditions being as follows: UNITEDSTATES N S fi The product obta ned contamed 97.7% we1ght n-paraf- 2514,580 7/1950 Hirschler 260666 ns and 0.7% weight aromat1cs. Thlsproduct was 253() 300 11/1950 hl washed with 4 x 2 /2 X vol. of 25% S0oleum to remove 2559517 7/1951 l the aromatics yielding a dearomatizedproduct containing 2914591 11/1959 B g s er 260676 98.4 n-parafiins and1.6% isoparaflins and naphthenes. 2935467 5/1960 Ffeck 1 260*671 Thismaterial was then reprocessed with a 5 A. sieve under 2938864 5/1960Fleck et 260676 the same conditions as previously and a n-parafiinfraction containing 99.9% weight of n-paraffins was obtained. 29719932/1961 Kimberhn et a1 260676 We claim: 2,988,502 6/1961 Ricards et al260676 1. A process for the production of n-paraifin frac- 31462778/1964 Haws et 260676 tions having a purity of at least 99% by weightcom- ALPHONSO D. SULLIVAN, Primary Examiner,

1. A PROCESS FOR THE PRODUCTION OF N-PARAFFIN FRACTIONS HAVING A PURITYOF AT LEAST 99% BY WEIGHT COMPRISING CONTACTING A PETROLEUM FRACTIONBOILING IN THE C10-C20 RANGE WITH A 5 A. MOLECULAR SIEVE AT ATEMPERATURE IN THE RANGE 300-450*C. AND A PRESSURE OF 0-300 P.S.I.G. TOPRODUCE AN INTERMEDIATE PRODUCT CONTAINING AT LEAST 95%, BY WEIGHT, OFN-PARAFFINS, SUBJECTING THE INTERMEDIATE PRODUCT TO A DEAROMATIZATIONOPERATION, CONTACTING THE DEAROMATIZED INTERMEDIATE PRODUCT WITH A 5 A.MOLECULAR SIEVE AT A TEMPERATURE IN THE RANGE 300450*C. AND AT APRESSURE OF 0-300 P.S.I.G. TO PRODUCE A FRACTION CONTAINING LESS THAN1.0% BY WEIGHT OF ISOPARAFFINS AND NAPHTHENES, AND RECOVERING AN-PARAFFIN PRODUCT THEREFROM HAVING A PURITY OF AT LEAST 99%.